Conditioned eyeblink response consists of two distinct components

The aim of these experiments was to obtain a detailed knowledge of how the orbicularis oculi muscle is activated during the execution of a conditioned eyeblink response (CR). This is the first critical step to understand the underlying neural mechanisms involved in the control of the CR. Decerebrate ferrets were trained in a classical conditioning paradigm. The conditioned stimulus (CS) was a train of electrical stimuli (15 pulses, 50 Hz, 1 mA) applied to the forelimb, and the unconditioned stimulus (US) was a train of electrical stimuli (3 pulses, 50 Hz, 3-4 mA) to the periorbital region. The CRs were studied by recording electromyograms (EMGs) from the orbicularis oculi muscle. The eyeblink CR in all animals showed a similar topography with at least two different components, CR1 and CR2, which were expressed at different rates. CR1 appeared first during acquisition, had a shorter onset latency, and was more phasic and more resistant to extinction than CR2. A marked pause in the muscle activity separated the two components. To control that the two-component CR were not species, paradigm or preparation specific, awake rabbits were trained with a tone CS (300 ms, 4 kHz, 64 dB) and a train of periorbital stimuli as US (3 pulses, 50 Hz, 3 mA). CR1 and CR2 were present in the rabbit eyeblink CR. The cerebellum is implicated in the control of CRs and to study whether separate neural pathways were responsible for CR1 and CR2, direct brachium pontis stimulation was used to replace the forelimb CS. CR1 and CR2 were present in the CR elicited by the brachium pontis CS. The presence of CR1 and CR2 after a unilateral lesion of the brachium conjunctivum shows that output from the contralateral cerebellar hemisphere was not the cause for any of the components. Other mechanisms that might be involved in the separation of the CR into two components are discussed. The results show that the eyeblink CR consists of at least two components, CR1 and CR2, which most likely originate either as a direct central command from the cerebellum or in the output pathway before the facial nucleus.